Following the progress of the illumination techniques, the electromagnetic ballasts are gradually replaced by the electronic ballasts to be applied to the illumination market. Comparing with the electromagnetic ballasts, the electronic ballasts have the advantages of small size, light weight, less flicker and longer lifetime and possess the competition power while they are applied to the illumination market so that the electronic ballasts have an extremely large development potential. In order to meet the IEC 61000-3-2 Class C regulation for the input current, the electronic ballasts having the PFC functions have been employed in the illumination apparatuses generally. Currently, the two-stage electronic ballasts having the PFC functions (as shown in FIG. 1) are the main-stream, and the two-stage electronic ballast comprises an AC input power source (supplying an AC input voltage Vin), a filter circuit (comprising an inductor LEMI and a capacitor CEMI), a rectifier circuit (comprising four diodes Dbr1-Dbr4), a PFC stage (comprising an inductor Lpfc, a diode Dy and a switch Spfc) and a full-bridge inverter (comprising an inductor Ls, two capacitors CB and Cp, four switches S1-S4 and a lamp) with the drawbacks of:
(1) requiring more elements, thus to have the relatively higher costs of the electronic circuit,
(2) requiring control ICs in both the PFC circuit and the power stage, thus to have the relatively complex circuit.
To solve the above-mentioned drawbacks, a half-bridge electronic ballast having a discontinuous-conduction-mode (DCM) charge-pump (CP) PFC circuit (as shown in FIG. 2) has been developed. Except for the AC input power source, the filter circuit and the rectifier circuit as shown in FIG. 1, the half-bridge electronic ballast having the DCM CP PFC circuit comprises a PFC stage (comprising a capacitor Cin and a diode Dy), a half-bridge inverter (comprising an inductor Ls, two capacitors CB and Cp, two switches S1-S2 and a lamp) and an output capacitor Cdc. However, the half-bridge electronic ballast having the DCM CP PFC circuit still has the following drawbacks:
(1) the circuit operating in DCM, such that the input current has a relatively larger di/dt, and the EMI is relatively more serious;
(2) the circuit having a relatively larger peak current, such that the circuit elements having a relatively larger withstand current must be used.
Due to that the half-bridge structure is not applicable in many medium and high power illumination applications, the present invention intends to combine the CCM CP PFC technique (see FIGS. 3 and 4) with the full-bridge electronic ballast. As shown in FIG. 3, the equivalent circuit of the CCM voltage-source CP PFC circuit has a rectified DC voltage source (supplying a rectified voltage |Vin| and a current Iin), two diodes Dx and Dy, an inductor Lpfc, a capacitor Cin, a voltage source (supplying a voltage Va), a load and an output capacitor Cdc, and a cross voltage of the output capacitor is an output voltage Vbus. As shown in FIG. 4, the equivalent circuit of the CCM current-source CP PFC circuit differs from FIG. 3 in that there is one diode Dx less, the rectified DC voltage source supplies a current Ipfc, the capacitor Cin has a cross voltage Vcin and is electrically connected to the diode Dy, the voltage source Va is replaced by a current-source (supplying a current Is), and the output capacitor Cdc is replaced by an output voltage source (supplying a DC output voltage Vbus). As a result, not only the circuit structure and the elements of the driving circuit can be simplified, but also the volumes of the PFC inductor and the input filter circuit can be reduced via the CP PFC circuit so as to decrease the circuit costs and to shrink the volume of the circuit. Due to that the circuit operates in the CCM, thus to have a relatively lower input current harmonic distortion, the relatively higher input current power factor, the relatively lower diode conduction losses and switching losses, so as to raise the overall circuit efficiency.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a full-bridge electronic ballast having a simplified continuous-conduction-mode charge pump PFC circuit.